In a heterodyne receiver, a broadband RF signal limited by an input band, pass filter is pre-amplified and then down-converted by an RF mixer. Tuning of a heterodyne receiver to a wanted reception frequency is done by changing the frequency of a local oscillator connected to the RF mixer. A narrowband filter connected downstream of the RF mixer filters out the intermediate frequency.
If an active mixer is used, the power of the RF signal input to the RF mixer must stay in the linear zone of the mixer and especially must not become higher than the input compression point. The input compression point depends on the gain of the RF mixer. It is known to detect the signal strength of the intermediate frequency after the narrowband filter and to use an analog control loop to adjust the gain of the pre-amplifier accordingly, thus keeping the signal strength of the RF signal input to the RF mixer in the linear zone. This works well for signal power changes inside the selected frequency band.
A digital audio broadband (DAB) system uses the frequency band III with frequencies comprised between 174 and 240 MHz and the L-band with frequencies between 1452 and 1492 MHz. Especially in the band III, the first and second out-of -band interferer can be up to 50 dB higher than the wanted signal. These interferers are too near to the wanted frequency band to be sufficiently attenuated by the input bandpass filter.
These out-of-band interferers can therefore overload the RF mixer in the heterodyne receiver. An overloaded mixer does not work in its linear zone anymore and the useful signal is not mixed and amplified properly. The interferers cannot be detected by monitoring the signal strength of the intermediate frequency because they are out-of-band.
Overloading of the RF mixer by out-of-band interferers can be prevented by placing a higher order bandpass filter in front of the RF mixer, but this is very expensive.